1. Field of the Invention
The present invention relates to an automatic focusing system for a scanning electron microscope equipped with a laser defect detection function, which is used to control the yield in semiconductor manufacturing processes.
2. Related Background Art
A major factor that leads to defective products in manufacturing super-highly integrated LSIs in recent years is said to result from by micro foreign matters that adhere to wafers. More particularly, the micro foreign matters turn into pollutants that cause breakage and short circuit of circuit patterns, which lead to a great extent to occurrence of defects in semiconductor chips and their lowered quality and reliability. With the miniaturization of circuit patterns, micro foreign matters having diameters on the order of 0.1 xcexm could be problematical. For this reason, it has become the most important objective, for controlling the process and improving yield in manufacturing super-highly integrated LSIs, to quantitatively and accurately measure, analyze and understand conditions of adhesion of micro foreign matters.
Currently, a particle examination apparatus shown in FIG. 1 is used for a method to quantitatively and accurately measure and analyze actual conditions of adhesion of micro foreign matters. This apparatus can display on a CRT 9 three types of images, {circle around (1)} a scanning electron microscope image, {circle around (2)} a high-sensitive dark-field image obtained by a high-sensitive CCD camera 4, and {circle around (3)} a dark-filed image or a bright-field image obtained by a CCD camera 5. A scanning electron microscope 1 is equipped with a lens barrel section 11, and a secondary electron detector (SED) 12. The lens barrel section 11 is equipped with a deflection device that deflects and scans an electron beam to be irradiated onto a sample 6. The SED 12 detects secondary electrons that are flipped out from the sample 6 by the electron beam. The detected data is associated with positions of the irradiated beam to be imaged, which provides the scanning electron microscope image {circle around (1)}.
A light source 8 emits light, which is passed through an optical path and is irradiated from above as a light spot on a surface of the sample. When the sample surface has a mirror surface, the light is uniformly reflected on the surface, but is scattered when there are discrete configurations that may be caused by foreign matters or the like. Based on the reflection or scattering of the light, the bright-field image {circle around (3)} is obtained by photographing the sample surface from above with the CCD camera 5.
A laser light source 7 emits a laser beam, which is diagonally irradiated from above on the sample surface. When the sample surface has a mirror surface, a total internal reflection of the laser beam occurs, but the laser beam is scattered at a foreign matter or the like that may be present on the sample surface. According to the total internal reflection or scattering of the laser beam, the dark-field image {circle around (3)} is obtained by photographing the sample surface from above with the CCD camera 5.
The high-sensitive dark-field image {circle around (2)} is basically the same as the dark-field image {circle around (3)}, but this is an image obtained by the high-sensitive CCD camera 4 that enables observation of even smaller particles. Because subjects are ultrafine particles, and scattered light caused by ultrafine particles is very weak, a high-sensitive CCD (ICCD) camera is used.
Detection of defects is conducted by this apparatus in the following manner. A microscope image {circle around (3)} or {circle around (2)} is photographed based on defect position information that has been obtained by another defect detection apparatus to obtain correct positional information of defects on a sample with respect to a reference position of the apparatus. Then, the defects are observed and analyzed by the scanning electron microscope. More specifically, first, positions of relatively large foreign matters with respect to a reference position of the apparatus are detected by using a bright-field image or a dark-field image of a relatively low magnification. The number and distribution condition of these defects may also be detected if required. To obtain a bright-field image, a control box 15 is operated with a computer 10 whereby a light source 8 is lit so that a surface of the sample is irradiated with a light spot from above, and the ordinary CCD camera 5 is used to photograph the image. To photograph a dark-field image, the control box 15 is operated with the computer 10 whereby a laser light source is lit to irradiate the surface of the sample with a laser beam, and the ordinary CCD camera 5 is used to photograph the image. The following examination is conducted for micro foreign matters whose presence could not be confirmed by the initial examination described above. The ICCD camera 4 with a higher sensitivity is used to photograph a high magnification dark-field image of the micro foreign matters. This enables detection of positions of the fine foreign matter particles that cannot be observed by the initial examination with respect to the reference position of the apparatus. The number and distribution condition of these micro foreign matters may also be detected if required. The scanning electron microscope is positioned based on the positional information of the target foreign matters, which is specified by the examination operations described above, and the target foreign matters are observed. Minute amounts of displacements that cannot be handled by the driving mechanism of the sample stage are treated as information on coordinates of the screen, which enables the deflection device of the electron optical system to handle such minute amounts of displacements. Although compositions of target foreign particles cannot be analyzed by a high-magnification optical image or a scanning microscope image obtained by detecting secondary electrons, they can be analyzed by using an electron microscope with a secondary X-ray detecting (EDS) function. This enables to specify causes of the defects more effectively.
However, in the defect detection method in which an optical microscope is used to observe scattered light of a laser beam, since the focal depth of the optical system is deep, defects can be detected to some degree even when the focal point of the optical microscope is not located on the wafer surface. However, since the scanning electron microscope has a shallow focal depth, it is difficult to observe defects in the size of 0.1 xcexcm when the focusing surface shifts by 10 xcexcm or more. For this reason, when defects are spotted with the laser beam, and such defects are to be observed by a scanning electron microscope, the defects may often move out of the focal point of the scanning electron microscope. In such a case, the defects (particles) cannot be observed. Moreover, since defects to be observed are extremely small, it would take a substantially long time to readjust the focus, which deteriorates the work efficiency.
The present invention relates to a function to maintain a focused condition without wasting time and labor in a system for observing and analyzing particle defects when positions of the defects specified by a dark-field image of an optical microscope are observed by a scanning electron microscope.
In accordance with an embodiment of the present invention, an automatic focusing function of a scanning electron microscope equipped with a laser defect detection function performs the steps of: correctly obtaining a shift (offset) amount between focal positions of an optical microscope and a scanning electron microscope; detecting at least one defect by a laser dark-field image of the optical microscope; then analyzing the dark-field image to readjust a focus of the optical microscope and adjust a height of the optical microscope; and automatically adjusting a focus of the scanning electron microscope by adding a readjusted amount of the focus of the optical microscope to the offset amount before an observation is conducted by the scanning electron microscope.
Also, the step of analyzing the dark-field image to readjust a focus of the optical microscope comprises a step of seeking a minimum defect area, a step of seeking a defect area having a brightest luminance, a step of seeking a defect area that provides a maximum differential value as a result of differential scanning of the dark-field image on a screen on the optical microscope, or a step combining at least two of the steps.
In the examination of particle defects, the automatic focusing function is capable of selecting one of a mode that extract one defect on the screen, a mode that extracts a plurality of defects on the screen, and a mode that extracts all defects on the screen.
Other features and advantages of the invention will be apparent from the following detailed description, taken in conjunction with the accompanying drawings that illustrate, by way of example, various features of embodiments of the invention.